Laser welded inkjet printhead assembly utilizing a combination laser and fiber optic push connect system

ABSTRACT

A method for the solderless laser welding of two materials by using a laser light beam attached to a fiber optic system which directs the light to a region where the laser beam can shine through one of the materials to create a seam weld. By using a fiber optic system the laser beam is converted into thermal energy and weld flaws due to underheating or destruction of the materials due to overheating do not occur. A seed-metal layer and weldable-metal patterns are formed on non-metallic materials-to-allow their laser welding by this method.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application relates to the subject matter disclosed in thefollowing U.S. Patents and co-pending U.S. Applications:

U.S. patent application Ser. No. 08/558,567, filed Oct. 31, 1995,entitled "Solderless Connection of Electrical Contacts UtilizingCombination Laser and Fiber Optic Push Connect System;"

U.S. patent application Ser. No. 08/550,328, filed Oct. 30, 1995,entitled "Inkjet Printhead with Trench and Backward Peninsulas;"

U.S. patent application Ser. No. 08/550,427, filed Oct. 30, 1995,entitled "Inkjet Printhead with Channels Connecting Trench and FiringChambers;"

U.S. patent application Ser. No. 08/131,816, filed Oct. 5, 1993,entitled "Inkjet Printhead Formed to Eliminate Ink Trajectory Errors,"which is a continuation-in-part of U.S. Pat. Nos.5,450,113, entitled"Adhesive Seal for an Inkjet Printhead."

U.S. Pat. No. 5,442,384, entitled "Integrated Nozzle Member and TABCircuit for Inkjet Printhead;" and

U.S. Pat. No. 5,278,584, entitled "Ink Delivery System for an InkjetPrinthead."

CROSS-REFERENCE TO RELATED APPLICATIONS

This application relates to the subject matter disclosed in thefollowing U.S. Patents and co-pending U.S. Applications:

U.S. patent application Ser. No. 08/558,567, filed Oct. 31, 1995,entitled "Solderless Connection of Electrical Contacts UtilizingCombination Laser and Fiber Optic Push Connect System;"

U.S. patent application Ser. No. 08/550,328, filed Oct. 30, 1995,entitled "Inkjet Printhead with Trench and Backward Peninsulas;"

U.S. patent application Ser. No. 08/550,427, filed Oct. 30, 1995,entitled "Inkjet Printhead with Channels Connecting Trench and FiringChambers;"

U.S. patent application Ser. No. 08/131,816, filed Oct. 5, 1993,entitled "Inkjet Printhead Formed to Eliminate Ink Trajectory Errors,"which is a continuation-in-part of U.S. Pat. Nos.5,450,113, entitled"Adhesive Seal for an Inkjet Printhead."

U.S. Pat. No. 5,442,384, entitled "Integrated Nozzle Member and TABCircuit for Inkjet Printhead;" and

U.S. Pat. No. 5,278,584, entitled "Ink Delivery System for an InkjetPrinthead."

The above patent and co-pending applications are assigned to the presentassignee and are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention generally relates to the adhesiveless laserwelding of two materials and, more particularly, to the laser welding oftwo materials using an optical fiber that holds the materials in contactwhile directing a laser emission to the pattern area to be welded.

BACKGROUND OF THE INVENTION

Thermal inkjet print cartridges operate by rapidly heating a smallvolume of ink to cause the ink to vaporize and be ejected through one ofa plurality of orifices so as to print a dot of ink on a recordingmedium, such as a sheet of paper. Alternatively, a piezoelectric elementmay be used to eject a droplet of ink through an associated orifice ontothe paper. The properly sequenced ejection of ink from each orificecauses characters or other images to be printed upon the paper as theprinthead is moved relative to the paper.

An inkjet printhead generally includes: (1) ink channels to supply inkfrom an ink reservoir to each ejection chamber proximate to an orifice;(2) a metal orifice plate or nozzle member in which the orifices areformed in the required pattern; and (3) a silicon substrate containing aseries of ink ejection elements, one ejection element per ink ejectionchamber.

To print a single dot of ink, an electrical current from an externalpower supply is passed through a selected thin film resistor. Theresistor is then heated, in turn superheating a thin layer of theadjacent ink within a vaporization chamber, causing explosivevaporization, and, consequently, causing a droplet of ink to be ejectedthrough an associated orifice onto the paper. Alternatively, apiezoelectric element may be used to eject a droplet of ink through anassociated orifice onto the paper.

In U.S. application Ser. No. 07/862,668, filed Apr. 2, 1992, entitled"Integrated Nozzle Member and TAB Circuit for Inkjet Printhead, " anovel nozzle member for an inkjet print cartridge and method of formingthe nozzle member are disclosed. This integrated nozzle and tab circuitdesign is superior to the orifice plates for inkjet printheads formed ofnickel and fabricated by lithographic electroforming processes. Abarrier layer formed on the substrate includes ejection chambers,surrounding each orifice, and ink flow channels which provide fluidcommunication between a ink reservoir and the ejection chambers. Aflexible tape having conductive traces formed thereon has formed in itnozzles or orifices by Excimer laser ablation. By providing the orificesin the flexible circuit itself, the shortcomings of conventionalelectroformed orifice plates are overcome. The resulting nozzle memberhaving orifices and conductive traces may then have mounted on it thesubstrate and barrier layer containing ink ejection elements associatedwith each of the orifices. Additionally, the orifices may be formedaligned with the conductive traces on the nozzle member so thatalignment of electrodes on a substrate with respect to ends of theconductive traces also aligns the ejection elements with the orifices.The leads at the end of the conductive traces formed on the back surfaceof the nozzle member are then connected to the electrodes on thesubstrate and provide energization signals for the ejection elements.The above procedure is known as Tape Automated Bonding ("TAB") of aninkjet printhead assembly, or TAB Head Assembly, (hereinafter referredto as a "THA")

An existing solution for attaching a nozzle member having orifices to asubstrate, containing ink ejection chambers and ink ejection elements,is to adhesively affix the top surface of the substrate's barrier layerto the back surface of the flexible circuit using a thin adhesive layer,such as an uncured layer of poly-isoprene photoresist, applied to thetop surface of the barrier layer. A separate adhesive layer may not benecessary if the top of the barrier layer can be otherwise be madeadhesive. The resulting substrate structure is then positioned withrespect to the back surface of the flexible circuit so as to align theink ejection chambers with the orifices formed in the flexible circuit.After the above-described preliminary adhesively-affixing step and thepreviously mentioned electrical-lead TAB bonding step, the aligned andbonded substrate and flexible circuit structure is then heated whileapplying pressure to cure the adhesive layer and firmly affix thesubstrate structure to the back surface of the flexible circuit.

The heat and pressure step utilizes an aluminum plate having arelatively malleable rubber shoe secured to the bottom surface of thealuminum plate. The heat and pressure step provides a downward force onthe aluminum plate while applying heat to the substrate in order toaffix the flexible tape to the top surface of the barrier layer. Therubber shoe extends over the edges of the substrate, and the downwardforce causes the tape to bend where not supported by the barrier layeror substrate. Due to the bending of the tape, the resulting TAB headassembly has nozzles which are skewed with respect to the substratecausing ink trajectory errors. Thus, when the TAB head assembly isscanned across a recording medium, the TAB head assembly trajectoryerrors will affect the location of printed dots and thus affect thequality of printing.

Nozzle skewing is caused by lamination pressure and the semifluidproperties of the polymeric barrier material at temperatures higher thanits glass transition temperature when heated. Delamination of the nozzlemember, from the barrier layer is caused by the post-bonding stress inthe barrier layer. During the lamination process, the barrier materialbetween the adjacent vaporization chambers is under pressure and iscompressed erratically, which causes sloping of the nozzle membersurface. A subsequent baking process releases stress in the barriercreated by the bonding process, increasing nozzle skewing, and causesdelamination. In a combined effect, skewing may also be caused by theevaporation of some volatile components in the barrier material andhence the barrier material and hence the barrier shrinkage at theexposed boundaries in the prolonged baking process. Delamination both atTHA level and pen level are caused by the weak adhesive interfacebetween the substrate and the flexible tape.

In addition, the barrier material and adhesion degrades with theaggressive solvents in newer ink formulations. Moreover, all workablesolutions developed for THA to print cartridge body detachment problemscaused by the stresses involved in the THA to print cartridge bodyattachment process, resulted in increased delamination at the adhesivesubstrate/tape interface.

Accordingly, it would be advantageous to have an improved printheaddesign for facilitating the attachment of a nozzle member to thesubstrate which increases the substrate/tape interface bond strength andreduces deformation of the barrier material, ink trajectory errors,detachment and delamination.

This in turn would result in ease of assembly, higher yields, improvedreliability, ease of surface serviceability, and overall material andmanufacturing cost reductions.

SUMMARY OF THE INVENTION

The present invention provides a method of laser welding of twomaterials by a laser beam having a specified wavelength, comprising thesteps of providing a first material with a seed metal pattern on asurface thereof and a second material with a corresponding mating seedmetal pattern on a surface thereof, said seed metal having a suitableabsorption at the wavelength of the laser beam; aligning the seed metalpattern on the surface of the first material and the correspondingmating seed metal pattern on the surface of a second material; holdingthe mating seed metal patterns in contact at a bond surface with anoptical fiber; and laser welding the seed metal patterns by directingthe laser beam through the optical fiber.

The present invention provides a method for the solderless laser weldingof two materials by using a laser light beam attached to a fiber opticsystem which directs the light to a portion of the material throughwhich the laser can propagate to create a seam weld. The phrases"through bulk material" and "through bulk substance", appearing incertain of the appended claims with regard to the path of the laserbeam, are used to make plain that in the invention recited in thoseparticular claims the beam is propagating through the material itself(of, e. g., a TAB tape or other component), not through a windowtherein. By using a fiber optic system the laser beam is optimallyconverted into thermal energy and weld flaws due to underheating ordestruction of the materials due to overheating does not occur. Themethod and apparatus provide rapid, reproducible laser welding even forthe smallest of contact geometries. For example, the method of theinvention results in solderless gold to gold compression laser weldingof a silicon substrate to the material contained in a polymer flexcircuit tape, such as a polyimide, without damaging the tape. A strongsolderless gold to gold bond can be formed between a gold bond line, orcomplex weld seam pattern on the flex circuit tape and a mating goldbond line or weld seam pattern on a semiconductor chip without anydamage to the tape and prevents delamination and nozzle skewingassociated with the adhesive curing process of other methods. Thereduced nozzle skewing provides less dot placement error for printcartridges, and therefore better print quality.

The present invention provides an improved printhead design forfacilitating the attachment of a nozzle member to the substrate which,increases nozzle area stiffness and nozzle camber angle anddirectionality, the substrate/tape interface bond strength and reducesdeformation of the nozzle member, which provides higher consistent inkchamber refill speeds, and reduces ink trajectory errors anddelamination. In addition, in particular embodiments the inventionprovides increased tolerance to aggressive solvents in inks and moreconsistent chamber geometry resulting in better control over dropvolume. The invention improves the quality, repeatability andreliability of the pen and results in less process steps, therebyimproving processing time, cost and reduced in-process handling.

The above in turn results ease of assembly, higher yields, improvedreliability, ease of nozzle serviceability, and overall material andmanufacturing cost reduction.

While the present invention will be described, for purposes ofillustration only, in conjunction with the laser welding of a TABcircuit to the silicon substrate of an inkjet printhead, the presentmethod and apparatus for the solderless laser welding of two contactmaterials by using a laser light beam attached to a fiber optic systemis applicable to laser welding other materials to each other.

Other advantages will become apparent after reading the disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention can be further understood by reference to thefollowing description and attached drawings which illustrate thepreferred embodiment.

Other features and advantages will be apparent from the followingdetailed description of the preferred embodiment, taken in conjunctionwith the accompanying drawings, which illustrate, by way of example, theprinciples of the invention.

FIG. 1 is a perspective view of an inkjet print cartridge according toone embodiment of the present invention.

FIG. 2 is a facial (unfolded flat) view of the front surface of the TapeAutomated Bonding TAB) printhead assembly ("THA") removed from the printcartridge of FIG. 1.

FIG. 3 is a perspective view of a simplified, schematic form of theinkjet print cartridge of FIG. 1. for illustrative purposes.

FIG. 4 is a facial (unfolded flat) view of the front surface of the TapeAutomated Bonding TAB) printhead assembly (hereinafter "TAB headassembly") removed from the print cartridge of FIG. 3.

FIG. 5 is a perspective view of the back surface of the TAB headassembly of FIG. 4 with a silicon substrate mounted thereon and theconductive leads attached to the substrate.

FIG. 6 is a side elevational view in cross-section taken along line A-Ain FIG. 5 illustrating one system (compare the above-mentionedapplication Ser.No. 08/558,567 for a more highly preferred system) forattachment of conductive leads to electrodes on the silicon substrate.

FIG. 7 is a top perspective view of a substrate structure containingheater resistors, ink channels, and vaporization chambers, which ismounted on the back of the TAB head assembly of FIG. 4.

FIG. 8 illustrates one process which may be used to form the preferredTAB head assembly.

FIG. 9 is a schematic diagram for a fiber push connect laser system asused in the present invention.

FIG. 10 shows in detail the flex circuit, a spot weld point, the TABlead and die pad.

FIG. 11 shows the temperature profile of the flex circuit, weld beam andspot weld location during the laser welding process with the FPC laser.

FIG. 12 shows the absorption property versus wavelength for variousmetals.

FIG. 13 illustrates the optical transmission results for five samples ofKapton® tape sputtered with 2, 5, 10, 15, and 25 nm of chromium.

FIG. 14 illustrates the temperature rise in flex circuits with Ti/W seedlayers.

FIG. 15 illustrates the temperature rise in flex circuits with achromium seed layer.

FIG. 16 illustrates temperature increase versus time in a 3-layer tapewith different thickness chromium seed layers.

FIG. 17 illustrates a first embodiment of the present invention.

FIG. 18 illustrates a variation of the FIG. 17 embodiment.

FIG. 19 illustrates a second embodiment of the present invention.

FIG. 20 illustrates a third embodiment of the present invention.

FIG. 21 illustrates a variation of the FIG. 20 embodiment.

FIG. 22 illustrates a fourth embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

While the present invention will be described, for purposes ofillustration only, in conjunction with the laser welding of a TABcircuit to the silicon substrate of an inkjet printhead, the presentmethod and apparatus for the solderless laser welding of two materialsby using a laser light beam attached to a fiber optic system isapplicable to laser welding of other types of electrical members to eachother.

Referring to FIG. 1, reference numeral 10 generally indicates an inkjetprint cartridge incorporating a printhead according to one embodiment ofthe present invention simplified for illustrative purposes. The inkjetprint cartridge 10 includes an ink reservoir 12 and a printhead 14,where the printhead 14 is formed using Tape Automated Bonding (TAB). Theprinthead 14 (hereinafter "TAB head assembly 14") includes a nozzlemember 16 comprising two parallel columns of offset holes or orifices 17formed in a flexible polymer flexible circuit 18 by, for example, laserablation.

A back surface of the flexible circuit 18 includes conductive traces 36formed thereon using a conventional photolithographic etching and/orplating process. These conductive traces 36 are terminated by largecontact pads 20 designed to interconnect with a printer. The printcartridge 10 is designed to be installed in a printer so that thecontact pads 20, on the front surface of the flexible circuit 18,contact printer electrodes providing externally generated energizationsignals to the printhead. Bonding areas 22 and 24 in the flexiblecircuit 18 are where the bonding of the conductive traces 36 toelectrodes on a silicon substrate containing heater resistors occurs.

In the print cartridge 10 of FIG. 1, the flexible circuit 18 is bentover the back edge of the print cartridge "snout" and extendsapproximately one half the length of the back wall 25 of the snout. Thisflap portion of the flexible circuit 18 is needed for the routing ofconductive traces 36 which are connected to the substrate electrodes,most particularly those which are connected through the far end window22. The contact pads 20 are located on the flexible c is secured to thiswall and the conductive traces 36 are routed over the bend and areconnected to the substrate electrodes through the windows 22, 24 in theflexible circuit 18.

FIG. 2 shows a face view of the TAB head assembly 14 of FIG. 1 whilestill flat and before attachment to the print cartridge 10 and prior towindows 22 and 24 in the TAB head assembly 14 being filled with anencapsulant. (In the most highly preferred configuration, these window22, 24 are omitted as set forth in the above-mentioned application Ser.No. 08/558,567) TAB head assembly 14 has affixed to the back of theflexible circuit 18 a silicon substrate 28 (not shown) containing aplurality of individually energizable thin film resistors. Each resistoris located generally behind a single orifice 17 and acts as an ohmicheater when selectively energized by one or more pulses appliedsequentially or simultaneously to one or more of the contact pads 20.

The orifices 17 and conductive traces 36 may be of any size, depth orthickness respectively and pattern, and the various figures are designedto simply and clearly show the features of the invention. The relativedimensions of the various features have been greatly adjusted for thesake of clarity.

The orifice 17 pattern on the flexible circuit 18 shown in FIG. 2 may beformed by a masking process in combination with a laser or other etchingmeans in a step-and-repeat process, which would be readily understood byone of ordinary skill in the art after reading this disclosure. FIG. 14,to be described in detail later, provides additional details of thisprocess. Further details regarding TAB head assembly 14 and flexiblecircuit 18 are provided below.

FIG. 3 is a perspective view of a schematic form of the inkjet printcartridge of FIG. 1, simplified for illustrative purposes. FIG. 4 is aview of the front surface of the Tape Automated Bonding (TAB) printheadassembly (hereinafter "TAB head assembly") shown flat and beforeattachment to the simplified print cartridge of FIG. 3.

FIG. 5 shows the back surface of the TAB head assembly 14 of FIG. 4showing the silicon die or substrate 28 mounted to the back of theflexible circuit 18 and also showing one edge of the barrier layer 30formed on the substrate 28 containing ink channels and vaporizationchambers. FIG. 7 shows greater detail of this barrier layer 30 and willbe discussed later. Shown along the edge of the barrier layer 30 are theentrances to the ink channels 32 which receive ink from the inkreservoir 12. The conductive traces 36 formed on the back of theflexible circuit 18 terminate in contact pads 20 (shown in FIG. 4) onthe opposite side of the flexible circuit 18 at location 38. The bondingareas 22 and 24 locate where the conductive traces 36 and the substrateelectrodes 40 (shown in FIG. 6) are bonded by using a laser light beamattached to a fiber optic system which directs the light to the locationto be bonded in accordance with the present invention.

FIG. 6 shows a side view cross-section taken along line A--A in FIG. 5illustrating the connection of the ends of the conductive traces 36 tothe electrodes 40 formed on the substrate 28. As seen in FIG. 6, aportion 42 of the barrier layer 30 is used to insulate the ends of theconductive traces 36 from the substrate 28. Also shown in FIG. 6 is aside view of the flexible circuit 18, the barrier layer 30, the bondingareas 22 and 24, and the entrances of the various ink channels 32.Droplets of ink 46 are shown being ejected from orifice holes associatedwith each of the ink channels 32.

FIG. 7 is a front perspective view of the silicon substrate 28 which isaffixed to the back of the flexible circuit 18 in FIG. 5 to form the TABhead assembly 14. Silicon substrate 28 has formed on it, usingconventional photolithographic techniques, two rows or columns of thinfilm resistors 70, shown in FIG. 7 exposed through the vaporizationchambers 72 formed in the barrier layer 30.

In one embodiment, the substrate 28 is approximately one-half inch longand contains 300 heater resistors 70, thus enabling a resolution of 600dots per inch. Heater resistors 70 may instead be any other type of inkejection element, such as a piezoelectric pump-type element or any otherconventional element. Thus, element 70 in all the various figures may beconsidered to be piezoelectric elements in an alternative embodimentwithout invalidating the herein-relevant general principles ofhydrodynamic operation of the printhead. Also formed on the substrate 28are electrodes 74 for connection to the conductive traces 36 (shown bydashed lines) formed on the back of the flexible circuit 18.

A demultiplexer 78, shown by a dashed outline in FIG. 7, is also formedon the substrate 28 for demultiplexing the incoming multiplexed signalsapplied to the electrodes 74 and distributing the signals to the variousthin film resistors 70. The demultiplexer 78 enables the use of manyfewer electrodes 74 than thin film resistors 70. Having fewer electrodesallows all connections to the substrate to be made from the short endportions of the substrate, so that these connections will not interferewith the ink flow around the long sides of the substrate. Thedemultiplexer 78 may be any decoder for decoding encoded signals appliedto the electrodes 74. The demultiplexer has input leads (omitted forsimplicity) connected to the electrodes 74 and has output leads (notshown) connected to the various resistors 70. The demultiplexer 78circuitry is discussed in further detail below.

Also formed on the surface of the substrate 28 using conventionalphotolithographic techniques is the barrier layer 30, which may be alayer of photoresist or some other polymer, in which is formed thevaporization chambers 72 and ink channels 80. A portion 42 of thebarrier layer 30 insulates the conductive traces 36 from the underlyingsubstrate 28, as previously discussed with respect to FIG. 4.

In order to affix the top surface of the barrier layer 30 to the backsurface of the flexible circuit 18 shown in FIG. 5, the substratestructure is positioned with respect to the back surface of the flexiblecircuit 18 so as to align the resistors 70 with the orifices formed inthe flexible circuit 18. This alignment step also inherently aligns theelectrodes 74 with the ends of the conductive traces 36. The top surfaceof the barrier layer 30 is then bonded to the back surface of theflexible circuit 18. The traces 36 are then bonded to the electrodes 74.These alignment and bonding processes are described in more detail withrespect to FIG. 8.

FIG. 8 illustrates one method for forming the TAB head assembly 14. Thestarting material is a Kapton® or Upilex® type polymer tape 104,although the tape 104 can be any suitable polymer film which isacceptable for use in the below-described procedure. Some such films maycomprise teflon, or polyimide, polymethylmethacrylate, polycarbonate,polyester, polyamide polyethylene-terephthalate or mixtures thereof.

The tape 104 is typically provided in long strips on a reel 105.Sprocket holes 106 along the sides of the tape 104 are used toaccurately and securely transport the tape 104. Alternately, thesprocket holes 106 may be omitted and the tape may be transported withother types of fixtures.

In the preferred embodiment, the tape 104 is already provided withconductive copper traces 36, such as shown in FIGS. 2, 4 and 5, formedthereon using conventional metal deposition and photolithographicprocesses. The particular pattern of conductive traces depends on themanner in which it is desired to distribute electrical signals to theelectrodes formed on silicon dies, which are subsequently mounted on thetape 104.

In the preferred process, the tape 104 is transported to a laserprocessing chamber and laser-ablated in a pattern defined by one or moremasks 108 using laser radiation 110, such as that generated by anExcimer laser 112. The masked laser radiation is designated by arrows114.

In a preferred embodiment, such masks 108 define all of the ablatedfeatures for an extended area of the tape 104, for example encompassingmultiple orifices in the case of an orifice pattern mask 108, andmultiple vaporization chambers in the case of a vaporization chamberpattern mask 108.

The laser system for this process generally includes beam deliveryoptics, alignment optics, a high precision and high speed mask shuttlesystem, and a processing chamber including a mechanism for handling andpositioning the tape 104. In the preferred embodiment, the laser systemuses a projection mask configuration wherein a precision lens 115interposed between the mask 108 and the tape 104 projects the Excimerlaser light onto the tape 104 in the image of the pattern defined on themask 108. The masked laser radiation exiting from lens 115 isrepresented by arrows 116. Such a projection mask configuration isadvantageous for high precision orifice dimensions, because the mask canbe physically remote from the nozzle member. After the step oflaser-ablation, the polymer tape 104 is stepped, and the process isrepeated.

A next step in the process is a cleaning step wherein the laser ablatedportion of the tape 104 is positioned under a cleaning station 117. Atthe cleaning station 117, debris from the laser ablation is removedaccording to standard industry practice.

The tape 104 is then stepped to the next station, which is an opticalalignment station 118 incorporated in a conventional automatic TABbonder, such as an inner lead bonder commercially available fromShinkawa Corporation, Model No. ILT-75. The bonder is preprogrammed torecognize an alignment (target) pattern on the nozzle member, create inthe same manner and/or step as used to created the orifices, and atarget pattern on the substrate, created in the same manner and/or stepused to create the resistors. In the preferred embodiment, the nozzlemember material is semitransparent so that the target pattern on thesubstrate may be viewed through the nozzle member. The bonder thenautomatically positions the silicon dies 120 with respect to the nozzlemembers so as to align the two target patterns. Such an alignmentfeature exists in the Shinkawa TAB bonder. This automatic alignment ofthe nozzle member target pattern with the substrate target pattern notonly precisely aligns the orifices with the resistors but alsoinherently aligns the electrodes on the dies 120 with the ends of theconductive traces formed in the tape 104, since the traces and theorifices are aligned in the tape 104, and the substrate electrodes andthe heating resistors are aligned on the substrate. Therefore, allpatterns on the tape 104 and on the silicon dies 120 will be alignedwith respect to one another once the two target patterns are aligned.

Thus, the alignment of the silicon dies 120 with respect to the tape 104is performed automatically using only commercially available equipment.By integrating the conductive traces with the nozzle member, such analignment feature is possible. Such integration not only reduces theassembly cost of the printhead but reduces the printhead material costas well.

The automatic TAB bonder then uses a gang bonding method to bond theconductive traces down onto the associated substrate electrodes asdescribed in U.S. patent application Ser. No. 08/558,567, filed Oct. 31,1995, entitled "Solderless Connection of Electrical Contacts UtilizingCombination Laser and Fiber Optic Push Connect System;" which is hereinincorporated by reference.

The tape 104 is then stepped to station 122 to affix the top surface ofthe barrier layer 30 to the back surface of the flexible circuit 18.During this step higher bond temperatures are generally preferred todecrease the bond time, but higher bond temperatures will soften theflex circuit and cause more deformation of the Kapton tape. Thus, it ispreferred to have higher temperature at the contact point and lowertemperature at the Kapton tape layer. This optimum contact temperatureprofile may be achieved by utilizing a Fiber Push Connect (FPC) singlepoint laser welding process. FPC in conjunction with a TAB circuitprovides an ideal solution for a TAB head assembly for an inkjet printerprinthead.

A schematic for a FPC laser system 200 is illustrated in FIG. 9. Thissystem consists of an Nd YAG or Diode laser 202, equipped with a glass(SiO₂) optical fiber 204. The system guides the laser beam to thecontact or attachment point 206 via the optical glass fiber 204. Anoptimum thermal coupling is achieved by pressing two parts together bymeans of the fiber 204 which creates a zero contact gap between the TABgold bond line 208 and die gold bond line 210 and thus improved thermalefficiency. FIG. 10 shows in greater detail the flex circuit 18, thecontact point 206, the TAB gold bond line 208 and die gold bond line210.

Referring to FIG. 9, a feedback temperature loop is achieved by means ofan infrared detector 212 through the glass fiber. The temperature orabsorption behavior response of the IR-radiation reflected by the goldbond lines 208, 210 at the contact point 206 is gathered. The outgoinglaser beam 220 from the laser source 202 goes through ahalf-transmission mirror or beam splitter 214 and through a focussinglens 216 into the glass fiber optic 204. The reflected light 218 fromthe fiber optic shown with dashed lines is reflected by the half mirror21 and arrives via focussing lens 222 at an IR detector 212 that isconnected to a PC Controller 224. The graph shown on the monitor 226 ofPC controller 224 is meant to show that the PC Controller 224 can storedefinite expected plots for the temperature variation of the laserwelding process with which the actual temperature variation can becompared. The PC Controller 224 is connected with the laser source 202so that the laser parameters can be controlled if necessary.

The reproducibility of a FPC laser bond depends both on a high degree ofthermal coupling between the gold bond lines 208, 210 and highabsorption of the laser energy by gold bond lines 208, 210. To optimizethe laser welding process, minimum absorption is desired in the Kaptontape and maximum absorption is desired in the gold bond lines 208, 210.Metals with higher absorption rate will transform a higher share of thelaser energy into heat. This will result in a shorter attach processwhich in turn will result in a higher quality bond.

The laser utilized is a YAG laser with a wavelength of 1064 nm. FIG. 12illustrates the absorption property versus wavelength for severalmetals. As can be observed from FIG. 12, chromium and molybdenum havethe highest absorption characteristics at this wavelength. Chromium wasselected as the base metal due to the fact that most flex circuitmanufacturers are using chromium as the seed layer. The penetrationdepth of the laser into chromium is about 10 nm with a spot size of 5nm, this requires a minimum chromium thickness of 15 nm. The laser beamcreates a localized heated zone causing the metals (or solder material),to melt and create a bond between two joining surfaces withoutincreasing the temperature of the Kapton tape. However, any gap betweentwo mating metal parts will cause overheating of the metal surfaceexposed to the laser beam. This will cause deformation of the flexiblecircuit 18 with no bond between metal surfaces. Also, an increasedtemperature in the flex circuit 18 will cause damage to the flexcircuit.

FIG. 11 illustrates a typical temperature profile of the flex circuit 18during laser welding process with the FPC laser. As it can be observedfrom FIG. 11, the temperature at the attachment area 206 is considerablyhigher than the Kapton tape 18 temperature. This is achieved due to thehigh degree of the transparency of the Kapton tape at differentwavelengths.

The Kapton polyimide tape is transparent to the YAG laser beam and thelaser beam passes through the 2 mil thick layer of polyimide withminimal absorption. Chromium is a conventional seed layer that is usedextensively to provide an adhesion layer between the copper trace andKapton polyimide in a two-layer flex circuit manufacturing process. Achromium layer with a minimum thickness of 10 nm (or 20 nm nominal) isrequired to provide a medium which absorbs the laser energy. Thethickness of the chromium layer varies depending upon the flex circuitmanufacturer, with reported thicknesses between 2 and 30 nm. A typicalflex circuit manufacturing process utilizes a thin layer (20 nm) ofsputtered chromium as a seed (adhesion) layer between the copper tracesand Kapton polyimide.

Five samples of the Kapton tape were sputtered with 2, 5, 10, 15, and 25nm of chromium, and optical transmission was measured for these samples.FIG. 13 illustrates the optical transmission results for these samples.It can be seen that optical transmission initially drops rapidly withincreased chromium thickness (from 65% for 2 nm of chromium, to 12% for15 nm of chromium), but optical transmission changes very slowly whenchromium thickness increases from 15 to 25 nm.

Laser welding process requires a fast temperature rise in the conductivetrace to minimize the temperature rise in the Kapton and thereforeminimize damage to the Kapton tape. FIGS. 14 and 15 illustratetemperature rise in several flex circuits with different constructions.FIG. 14 illustrates temperature rise in flex circuits with thicker seedlayers. It is important to notice that flex circuits with 10 nm or lessof Ti/W did not reach the temperature that is required for gold/goldlaser welding, but the flex circuit with 20 nm of Ti/W did reach thelaser welding temperature. Also, it should be noted that the rise timein the flex circuit with thicker Ti/W is faster, minimizing thepotential of damage due to high localized temperatures in the Kaptontape.

The temperature (IR-Signal) fluctuation in the flex circuit with 20 nmof Ti/W is indicative of the fact that this flex circuit reached themaximum preset temperature required for gold/gold laser welding and thenthe laser feed-back loop temporarily dropped the laser energy so thatincrease in the TAB bond temperature did not damage the Kapton tape. Assoon as the temperature of the Kapton tape dropped (by a preset amount),the laser energy automatically increased to full power to increase theTAB gold bond line temperature, and created a reliable gold/gold bond.

FIG. 15 illustrates similar results for different flex circuits with achromium seed layer as opposed to Ti/W seed layer. It can be observedthat flex circuit with 10 nm of chromium did reach the presettemperature required for gold/gold welding Therefore, chromium seedlayer has higher absorption characteristics compared to Ti/W seed layerfor a YAG laser.

FIG. 16 illustrates temperature increase versus time in a 3-layer tapewith a 20 nm chromium layer, a tape with a 5 nm chromium layer, and atape with no chromium layer. As can be seen in FIG. 16, only the flexcircuit with a 20 nm chromium layer indicated a rapid temperature rise.

Since it was established that chromium thickness is essential to theintegrity of the gold/gold laser bond, when a YAG laser is used, anoptimum chromium thickness was selected as a base line. Referring toFIG. 13, a chromium thickness over 15 nm does not decrease transmissionimportantly. Based on FIG. 15, a chromium thickness of 10 nanometers isthe absolute minimum required thickness to provide a successful laserbond. FIG. 15 also illustrates that a flex circuit with 15 nm ofchromium exhibits a much faster temperature rise in the copper trace,resulting in less or no damage to the Kapton tape. Therefore, 15 nm ofchromium is optimum to provide a reliable and repeatable laser bond.

Some chromium diffusion into the copper is expected during thesputtering of chromium as a seed layer and plating processes duringmanufacture of the flex circuits. Diffusion of the chromium into thecopper is a time and temperature dependent process, and it is difficultto determine the amount of chromium that will be diffused into thecopper during these processes. Normally, it is estimated that a maximumamount of diffused chromium is under 5 nm. Based on these factors, aminimum chromium thickness after the sputtering process was establishedas 20 nm. This thickness should guarantee a minimum chromium thicknessof 15 nm after the completed manufacture of the flex circuit.

FIGS. 17 to 22 show various embodiments of the present invention. Ineach of FIGS. 17-22, (A) shows the barrier structure, including inkvaporization chambers 72 and chamber walls 71, 73, and resistors 70; (B)shows a cross-section taken along line B--B of (A); and (C) shows theweld seam pattern for the embodiment.

In a first embodiment shown in FIG. 17, the walls 71, 73 of thevaporization chamber 72 are laid down in gold on substrate or die 28.The material of the barrier layer 30 is completely replaced with gold toconstruct the ink ejection chamber 72. Thus, the gold thickness is ofthe order of 15 μm with the exact value depending on the printheadarchitecture. The side walls 71 and rear wall 73 of the vaporizationchamber 72 form a gold weld line pattern 210 on the barrier layer 30. Acorresponding mating gold weld line pattern 208 is laid on the flexcircuit 18 to match the outline or area of the vaporization chamber 72.This weld seam 208 is made of copper laid on a chromium seed layer andcoated with gold to prevent corrosion. The substrate 30 and flex circuit18 are precisely aligned and welded using the FPC laser welding systemand technique as described above. The position of the FPC probe is movedalong the weld seam, to create a weld line, by using overlappingpatterns of spot welds. The advantages of this embodiment are,resistance to corrosion and delamination resulting in better reliabilityand better stiffness, resulting in better dot placement and refillperformances, thereby resulting in better print quality.

A modified form of this embodiment is shown in FIG. 18. In thisvariation the top 1 to 2 μm of the barrier structure 30 is replaced with1 to 2 μm of gold. A weld seam pattern 208 is laid on the flex circuit18 to match the outline of the vaporization chamber 72. This weld seam208 is made of copper laid on a chromium seed layer and coated with goldto prevent corrosion. The substrate 28 and the flex 18 are preciselyaligned and welded using the FPC welding system. The position of the FPCprobe is moved along the weld seam, to create a weld line, by usingoverlapping patterns of spot welds. The main advantage of thisembodiment is better stiffness, resulting in better dot placement andrefill performances, thereby resulting in better print quality.

In a second embodiment shown in FIG. 19, the walls 71, 73 of thevaporization chamber 72 are again laid in gold on substrate or die 28.However, in this case the height of the vaporization chamber 72 on thesubstrate is in the order of 1 to 2 μm. This embodiment is advantageousif plating up to the wall 71 height thicknesses of approximately 15microns are difficult for the manufacturing process to handle. Again thepattern of the vaporization chamber walls 71, 73 laid out in gold on thesurface of the barrier layer 30 forms a gold weld line pattern 210. Theremainder of the vaporization chamber 72 is formed by ablating a recesson the flex 18 to match the ink chamber layout of the gold barrier. Acorresponding mating gold weld seam pattern 208 is laid on the flexcircuit 18. This weld seam is made of copper laid on a chromium seedlayer and coated with gold to prevent corrosion. The substrate 30 andthe flex 18 are precisely aligned and welded using the FPC weldingsystem. The position of the FPC probe is moved along the weld seam, tocreate a weld line, by using overlapping patterns of spot welds. Theadvantages of this embodiment are resistance to corrosion anddelamination resulting in better reliability, better stiffness,resulting in better dot placement and refill performances, therebyresulting in better print quality.

In a third embodiment shown in FIG. 20, the side walls 71 of thevaporization chambers 72 are made of the barrier material 30 and thecritical back wall 73 is fabricated completely out of gold. Acorresponding mating gold weld seam pattern 208 is laid on the flexcircuit 18 to match the outline of the vaporization chamber back wall73. This weld seam is made of copper laid on a chromium seed layer andcoated with gold to prevent corrosion. The substrate 30 and flex circuit18 are precisely aligned and welded using the FPC laser welding system.The position of the FPC probe is moved along the seam, to create a weldline, by using overlapping patterns of spot welds. The advantages ofthis embodiment are resistance to corrosion and delamination resultingin better reliability and, better stiffness, resulting in better dotplacement and refill performances, thereby resulting in better printquality.

A variation of this embodiment shown in FIG. 21, is to build only thetop 1 to 2 μm of the back wall 73 barrier structure with gold. A weldseam pattern 208 is laid on the flex circuit 18 to match the outline ofthe vaporization chamber back wall 73. This weld seam is made of copperlaid on a chromium seed layer and coated with gold to prevent corrosion.The substrate and the flex are precisely aligned and welded using theFPC welding system. The position of the FPC probe is moved along theweld seam, to create a weld line, by using overlapping patterns of spotwelds. The main advantage of this embodiment is better stiffness,resulting in better dot placement and refill performances, therebyresulting in better print quality.

In a fourth embodiment shown in FIG. 22, the entire vaporization chamber72 is built out of barrier material, but a straight gold baffle bar 77is formed behind the back walls 73 of the vaporization chambers 72 toform gold weld line pattern 210. This baffle structure 77 is builtslightly behind the back wall 73. This baffle structure 77 can befabricated either by placing a thin 1-2 μm gold weld seam pattern 210 onthe substrate 30 and a thick (same thickness as the vaporization chamber72) weld trace on the flex 18 or by laying a thick gold structure (samethickness as the vaporization chamber 72) on the substrate 30 and a 1-2μm weld seam line 208 on the flex 18. The thickness in traces can alsobe shared between the substrate 30 and the flex 18 to meet specificdesign rules for the product and process. The main advantage of thisembodiment is better stiffness, resulting in better dot placement andrefill performances, thereby resulting in better print quality.

In each of the above embodiments, the FPC laser is used to form a fullweld along the gold weld lines or weld patterns 208, 210. Experimentalresults show that average peel strengths using FPC laser welding are18.3 lbs. versus 3.6 lbs. for conventional adhesive bonding.

Thereafter the tape 104 steps forward and is optionally taken up on thetake-up reel 124. The tape 104 may then later be cut to separate theindividual TAB head assemblies from one another.

The resulting TAB head assembly is then positioned on the printcartridge 10, and the previously described adhesive seal 90 is formed tofirmly secure the nozzle member to the print cartridge, provide anink-proof seal around the substrate between the nozzle member and theink reservoir, and encapsulate the traces in the vicinity of theheadland so as to isolate the traces from the ink.

Peripheral points on the flexible TAB head assembly are then secured tothe plastic print cartridge 10 by a conventional melt-through typebonding process to cause the polymer flexible circuit 18 to remainrelatively flush with the surface of the print cartridge 10, as shown inFIG. 1.

The foregoing has described the principles, preferred embodiments andmodes of operation of the present invention. However, the inventionshould not be construed as being limited to the particular embodimentsdiscussed. As an example, while the present invention was described inconjunction with the laser welding of a TAB circuit to the siliconsubstrate of an inkjet printhead, the present method and apparatus forthe solderless laser welding of two materials by using a laser lightbeam attached to a fiber optic system is applicable to laser welding ofother types of materials to each other. Likewise, while the presentinvention was described in conjunction with solderless gold to goldlaser welding of two materials to each other, the present method couldbe used for the solderless laser welding using other conductive metals.Thus, the above-described embodiments should be regarded as illustrativerather than restrictive, and it should be appreciated that variationsmay be made in those embodiments by workers skilled in the art withoutdeparting from the scope of the present invention as defined by thefollowing claims.

What is claimed is:
 1. A method of connection of two nonmetallicmaterials by laser welding, using a laser beam having a specifiedwavelength, comprising the steps of:providing a first nonmetallicmaterial with a first weldable-metal pattern on a surface thereof;providing a second nonmetallic material with a seed-metal pattern on asurface thereof and a second weldable-metal pattern on the seed-metalpattern, said seed metal having a suitable absorption at the wavelengthof the laser beam; wherein at least one of the weldable-metal patternsis not for conduction of electricity after welding is complete;substantially aligning the weldable-metal patterns on the surfaces ofthe first material and the second material; using an optical fiber tohold the weldable-metal patterns in contact at a bond surface; and laserwelding the metal patterns together by directing the laser beam throughthe optical fiber.
 2. The method of claim 1, wherein:the laser beam haswavelength in the near infrared; and the second providing step comprisesusing chromium to form the seed-metal pattern.
 3. The method of claim 2,wherein:the directing step comprises directing the laser beam to theseed-metal pattern through bulk substance of the second nonmetallicmaterial.
 4. The method of claim 2, wherein:the laser welding stepcomprises using an yttrium-aluminum-garnet laser to form the laser beam.5. The method of claim 2, wherein:the second providing step provides aseed-metal pattern at least ten nanometers thick.
 6. The method of claim1, wherein:the directing step comprises directing the laser beam to theseed-metal pattern through bulk substance of the second nonmetallicmaterial.
 7. The method of claim 1, further comprising the stepof:before the welding step, substantially aligning the weldable-metalpatterns on the surfaces of the first and second materials respectively.8. The method of claim 1, wherein:the two material-providing steps bothprovide substantially identical weldable metals; whereby the laser beamliterally welds two layers of a common metal, as distinguished fromsoldering.
 9. The method of claim 1, wherein:the two material-providingsteps both provide patterns of gold.